Isophorone nitrile (IPN) or 3-cyano-3,5,5-trimethylcyclohexanone is a critical intermediate in the synthetic scheme to the diamine (IPDA) and finally to the diisocyanate (IPDI). Numerous prior art processes have been developed to synthesize IPN. For example, U.S. Pat. No. 4,299,775 to Dubreux discloses a two-phase process for the preparation of IPN by reacting isophorone (IPH) with a cyanide in the presence of a catalytic amount of a phase-transfer agent. The quaternary ammonium catalysts are used as their chloride or bromide salts; the chloride ion or bromide ion is exchanged for cyanide ion and this cyanide ion is transferred from the water to the solvent layer via the well known phase transfer mechanism.
German Patent No. 1,240,854 to Scholven-Chemie discloses a process for the preparation of IPN by reacting isophorone with hydrogen cyanide in the presence of a basic catalyst such as alkali cyanide, hydroxides or alcoholates. The catalyst is removed by washing with dilute nitric acid.
Japanese Laid-Open specification 61-33157 to Nippon Kagako K.K. discloses a process for the preparation of isophorone nitrile by reacting isophorone with hydrogen cyanide in the presence of tetra-n-butylammonium hydroxide, tetra-n-butylphosphonium hydroxide or benzyltrimethylammonium hydroxide. The resulting reaction liquid is washed with water.
However, acidic washing to remove catalyst produces an aqueous effluent saturated in isophorone and containing some cyanide. Disposal of such an effluent adds significantly to the cost of the product.
European Patent Application 0 433 615 discloses a method for producing 1,3,3-trimethyl-5-oxo-cyclohexane-carbonitrile by the addition of hydrogen cyanide to isophorone in the presence of lithium hydroxide as a catalyst, at temperature conditions of 100.degree. to 160.degree. C. U.S. Pat. No. 5,011,968 discloses a process for producing isophorone nitrile in the presence of a quaternary ammonium hydroxide as a catalyst.
Many of the conventional catalysts, and in particular, amine-generating catalysts, used in the isophorone nitrile production can poison the metal catalysts used in the subsequent hydrogenation of isophorone nitrile to IPDA, thereby significantly reducing catalyst life. It is therefore advantageous and often necessary to eliminate such catalysts from the product, such as by distillation. However, such additional process steps are labor intensive and costly. In addition, important by-products of the base-catalyzed reaction of HCN and IPH include diisophorone (dimer) and its conversion to cyano derivatives. For example, as described in the Journal of Organic Chemistry, 42, (9), 1600-1607 (1977), the following reactions can occur: ##STR1##
Isophorone itself contains many impurities. Upon exposure to air, the product becomes yellow, which is believed to be a result of the apparent oxidation product of isophorone via direct addition of oxygen to the unsaturated site:
Thus, it is also advantageous to minimize the formation of such impurities, of diisophorone, and of its HCN adducts. In addition, elimination of residual HCN and excess IPH from the resulting product is necessary.